The invention finds applicability in producing more effective additives for cement and concrete formulations. More specifically fly ash is improved for use as an additive to cement.
Large quantities of coal are burned world wide. Typically, coal is ground to a fine powder, conveyed to the boiler and burned to produce steam to power turbines which generate electricity. A by-product of this coal combustion is a fly ash residue. During the combustion process inert clay and shale minerals in the fly ash react with lime to form cementacious materials. This pozzolanic property of fly ash is exploited. That is, fly ash incorporated into cement reacts with free lime during the concrete forming process to produce a cement forming a stronger concrete. Using fly ash as a pozzolan in concrete saves on the amount of cement to be used. There is a caveat however to using fly ash in concrete. The American Society for Testing and Material specifies a carbon content in fly ash of less than 6% as being useable in cement. It would be advantageous to employ fly ash having higher concentrations of carbon as a fly ash additive in cement to make concrete. The herein disclosed invention addresses this problem and offers a solution.
It is known in the art to treat fly ash containing higher levels of carbon with ozone to make the fly ash with a higher level carbon useable in cement. This ozone treatment of fly ash is referred to in the art as pacification. When pacifying fly ash, the prior art has employed an ozone generated by an external source. The elimination of this prior art external ozone generation system is a main goal of the present invention. As described, reacting ozone with high carbon fly ash produced by the combustion of fossil fuels will pacify the carbon and make it possible to use high carbon fly ash as a concrete additive.
The herein disclosed invention recognizes that processes designed to reduce carbon in fly ash are often remiss in not effectively reducing the carbon content to acceptable levels. The herein disclosed invention is designed to remedy the problem of insufficient or inefficient carbon reduction of fly ash. An example of a process in which carbon in fly ash may not be sufficiently reduced is in a triboelectric process for reducing carbon in fly ash.
1. Objects of this Invention
An object of this invention is to efficiently produce a fly-ash product which will have a beneficial effect on concrete.
A further object of this invention is to conveniently produce a fly ash composition allowing a higher carbon-content fly ash and acceptable levels of air entrainment agents to be used in concrete.
A significant object of this invention is a method for producing ozone treated fly ash in the exhaust gas stream during coal combustion.
A main object of this invention is to pacify fly ash with unacceptably high levels of carbon produced in triboelectric process, however, this invention could be used in any pneumatic system that is used to transport fly ash.
These and other objects of the present invention will become apparent from a reading of the following description taken in conjunction with the enclosed drawings.
2. Prior Art Patents
Kanter et al (U.S. Pat. No. 4,657,738) teaches a stack gas emissions control system for electrostatically removing pollutants from the gas.
Batchelor (U.S. Pat. No. 4,657,738) teaches the production of ozone using the corona discharge between electrodes.
Bittner et al (U.S. Pat. No. 6,074,458) teaches an apparatus for separating carbon particles from fly ash in which a triboelectric separator is employed.
Marketing fly ash for use as an additive in concrete is an important economic issue for the utility industry. However, the presence of unburned carbon in fly ash can limit fly ash use as a concrete additive. The limitation on the use of fly ash is not caused by the adverse affect that carbon has on concrete properties, but rather by the affect carbon has on air entrainment agents that are used in concrete mixtures. Carbon reacts with these agents rendering them ineffective as air entrainment agents. The affect becomes more pronounced and more sensitive to carbon concentration as the carbon concentration increases. The problem can be overcome by adjusting the amount of air entrainment agent frequently to match the carbon concentration of the fly ash, but this adds a level of complexity to the process that is not acceptable to concrete manufacturers. Generally, fly ash with a carbon content of greater than 3% cannot be sold in today""s market as an additive for concrete.
Recent research indicates that exposing the surface of carbon in fly ash to ozone will xe2x80x9cpacifyxe2x80x9d the surface of the carbon so that it no longer reacts with commonly used air entrainment agents. This discovery means that ozone-treated ash with a higher carbon content could be used in concrete without the need to increase the amount of air entrainment agent. In other recent research, it has also been discovered that a low carbon ash can be produced by using a triboelectric ash separation process. This process produces a stream of xe2x80x9cproductxe2x80x9d ash lower in carbon than the parent ash. This ash product with lower carbon content leaves the separation chamber entrained in a transport gas stream. As a result of this separation process, the ash particles carry a significant negative charge. It is pointed out, however, that the ash produced in the triboelectric process while reducing carbon content, will often times still have an unacceptably high carbon content. The herein disclosed invention describes a method of adding ozone to the entrained ash/carrier gas stream to pacify the unacceptably high carbon-content remaining in the ash after triboelectric separation. This approach makes it possible to relax the upper limit of the carbon in the ash and, thereby, increase the fraction of saleable ash produced by the separation process. In fact, adding ozone to the outlet product stream by any means will accomplish this goal. The ozone could be generated externally and injected into the piping that carries the ash from the separation chamber to the storage silo. The herein disclosed invention eliminates the need for external generation and injection of ozone.
In the normal combustion of coal, fly ash particles are produced. These fly ash particles contaminated with carbon are entrained in an air stream that flows through an exhaust pipe. The mixture of ash/carbon has a net negative charge. As part of the improvement of the herein disclosed invention, metal plates, electrically insulated from the pipe are inserted into the exhaust pipe. These plates are provided with sharp metal spikes attached thereto. As the negatively charged ash particles strike the plate, they impart a charge to the surface of the plate. In time, sufficient charge will accumulate on the plate to produce a substantial electric field around the plate. The electric field will be intensified at the tips of the sharp spikes, and when the charge on the plate is high enough, a corona discharge will begin at the tips of the spikes. The corona discharge will produce ozone. This discharge will bleed charge from the plate, and a steady-state condition will be established, whereby the charge deposited on the plate by ash particles that continue to strike the plate will be matched by the charge leaving the plate through the corona discharge. That is, an equilibrium condition will be reached wherein the rate of charge loss through the corona discharge will equal the rate of charge deposition due to turbulent charged particle impact.
It is well known that a negative corona in air produces ozone. In fact, some commercial ozone generators use a negative corona discharge to produce ozone. Hence, the corona discharge in the pipe transporting the air with entrained, charged ash particles will spontaneously generate ozone. Adding insulated metal plates with spikes in the exhaust pipe will eliminate the need for an external ozone generator to supply ozone to high carbon content fly ash.
The herein disclosed invention discloses alternate processes that do not require the use of an external ozone generator. In a first embodiment a number of metal structures are inserted into the transport pipe in such a manner that they are electrically insulated from the transport pipe. Each metal structure has one or more sharp edges or points with a very fine radius. As the charged particles in the pipe strike the metal, they deposit a charge. This process will, in time, create a voltage potential between the metal surface and surrounding environment. When this potential is high enough, a corona discharge will occur at the shaped edges of the metal structures that will spontaneously generate ozone.
Depending on the charge levels of the entering particles, the natural level of ozone generation may or may not be great enough to pacify the carbon in the ash. If the spontaneously generated charge and electric potential is insufficient, the metal structure can be connected to an external high-voltage power supply to enhance the rate of ozone generation. The use of a dielectric barrier on the discharge elements can be used to further enhance the rate of ozone generation. In either case, the sharp-edged components of the metal structure can be placed in protected flow zones to minimize fly ash abrasion. Therefore, only a hardened metal surface is exposed to ash impact. This approach will produce a simple, inexpensive system that will not require frequent replacement.
There are a number of patents covering the triboelectric ash beneficiation processes (to be supplied later). None of them relate to the use of ozone or any other agent to pacify the excessively high carbon remaining in ash.
As previously pointed out as an additive to cement, fly ash with a carbon content of 3% or less is desired. However, this low concentration of carbon is not easy to obtain or if obtainable is expensive and not economically feasible. One way of enabling higher carbon content fly ash to be used as a cement additive is to treat the higher carbon-content fly ash with ozone. A first embodiment of this invention entails taking a fly ash with a reduced carbon, but not a carbon content reduced sufficiently low to be a useful for adding air entrainment agents. That is, the carbon content of the fly ash is above the 3% acceptable level for adding air entrainment agents. At times the triboelectric separation process will produce fly ash of a carbon content of over 3%. In such cases adding ozone in situ to the higher carbon fly ash product by corona discharge will produce a useable product. Another embodiment of this invention contemplates use of insulated spiked metal plates as an ozone generator in conventional combustion systems generating ozone-treated fly ash by-product.
Said in another way the herein disclosed invention is directed to a process for treating negatively charged fly ash particles with unacceptably high levels of carbon which cannot be economically used as an additive to cement comprising turbulently subjecting in a gas stream containing said negatively charged fly ash particles with unacceptably high levels of carbon to a metal plate having at least one metal spike thereon to produce ozone through corona discharge wherein the ozone causes pacification of the fly ash with unacceptably high levels of carbon so that the fly ash can be efficiently used as an additive to cement. The method involves ozone produced by corona discharge in an exhaust tube, but instead of a tube any closed area wherein fly ash turbulence takes place would be operative.
In its broadest aspect the herein disclosed invention contemplates an electrostatic device producing ozone through corona discharge comprising sharp-tipped components placed on a conductive plate with an insulated bottom to be placed in a gas flow pipe. The sharp edged component can be a spike(s), wires or like metal pointed devices. The number of sharp edged components is not critical so long as adequate corona discharge to produce adequate amounts of ozone takes place.